In image forming apparatus such as laser printers, photocopiers, ink jet printers and the like, it is well known to pass imaged media through a fusing nip to fuse the image into the media. The fusing nip is typically created by biasing two rollers together. In some fusing systems one or both rollers are positively heated. In those systems having one roller that is positively heated, the side of the media containing the unfused toner or ink image is contacted by the heated roller to fuse the image into the media. The fusing process thus utilizes a combination of elevated temperature and pressure within the fusing nip to accomplish the desired amount of image fusing.
In some cases the fusing rollers will have different hardnesses. For example, the heated roller may have a lower hardness than the pressure (non-heated) roller. This nip geometry creates a nip-induced curvature or stress in the media that causes the media to bend or curl toward the pressure roller upon exiting the fusing nip. This nip geometry favors media release from the heated roller by directing the media away from the heated roller. The fusing nip may also have an opposite geometry, such that the pressure roller has a lower hardness than the heated roller. This nip geometry tends to favor duplex printing by bending or stressing the media toward the harder heated roller upon exiting the fusing nip. In this manner, the first image side of the media that is contacted by the pressure roller during its second pass through the fusing nip is directed away from the pressure roller to avoid image offset and to prevent the media from wrapping around the pressure roller.
Media speed within the fusing nip is governed by the roller surface velocities within the nip ("nip velocity"). The surface velocity of a roller is the combination of the roller free surface velocity and the roller surface deformation rate within the nip. Media wrinkling can be caused by any differential nip velocities across the media width. For example, if the rollers drive the center of the media faster than the media edges, or one edge faster than the other edge, media wrinkling may occur. Reducing the variation of nip velocity across the roller length has proved effective in reducing media wrinkling in most types of media. However, envelope wrinkling may still occur even when the differential nip velocity is at a minimal level.
In a fusing nip formed by rollers having different hardnesses, the surface velocity of the softer roller will be greater than the surface velocity of the harder roller due to the surface deformation of the softer roller. In the case of an envelope being transported through the nip, this nip geometry causes the envelope surface adjacent to the soft roller to move faster than the envelope surface adjacent to the hard roller. This creates a relative slip between the top and bottom layers of the envelope. Because these layers are affixed around their periphery and thereby prevented from slipping relative to one another, the differential velocities create increasing stress at the trailing end of the envelope as the envelope moves through the fusing nip. This stress creates wrinkles at the trailing end of the envelope.
The present invention seeks to reduce media wrinkling, especially in envelopes, caused by transport through a fusing nip by providing a method and apparatus for reducing stress build up in the media as it travels through the fusing nip. To compensate for the nip stress created by differential roller surface velocities, the media is bent or "pre-stressed" in a direction opposite to the nip stress prior to the media entering the fusing nip. A media support surface is positioned upstream from the fusing nip to "pre-stress" the media by providing pre-nip bending to the media. The end of the support surface nearest the nip is spaced from the nip-induced curvature of the media. In this manner, the media is bent prior to entering the nip in a direction opposite to the nip-induced curvature of the media. This pre-nip bending of the media substantially offsets the nip-stress imparted on the media by the fusing nip to avoid wrinkling.